Process for making resistors



Nov. 13, 1962 w. M. KoHRiNG 3,063,100

PROCESS FOR MAKING RESISTORS Filed Sept. 21, 1959 JNVENToR. FIG. 5wlLauR M. KoHRlNG s n 191'.. r Ml had* y ATTORNEYS United States PatentOffice Patented Nov. 13, 1962 3,063,100 PRUCESS FOR MAKING RESESTRSWilbur M. Kohring, 3318 W. 159th St., Cieveland 11, @trio Filed Sept.Z1, 1959, Ser. No. 841,315 4 Ctairns. (Ci. 118-59) The invention ingeneral relates to resistors and more particularly to protectivecoatings for resistors and the process for making same.

An object of the invention is to provide a resistor with a protectivetubular shield and the process for making same.

Another object of my invention is to provide a resistor with at least aprotective coating and a tubular shield and the process for making same.

Another object of the invention is to provide a resistor with at least aprotective coating and a tubular shield with the coating and the shieldcontaining a silicone matcrial and the process for making same.

Another object of the invention is to provide a resistor with aprotective shield comprising a hollow tube.

Another object of my invention is to provide a resistor with aprotective shield comprising a hollow silicone tube.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims, taken in conjunctionwith the accompanying drawing, in which:

FIGURE l shows a longitudinal View of a resistance -unit embodying thefeatures of my invention, partly in section;

FlGURES 2 and 3 show the steps by which my invention is constructed;

FIGURE 4 shows a section of a hollow silicone rubber tube; and

FIGURE 5 shows a method of sliding a rubber tube over a resistorelement.

With reference to FIGURE l, my invention comprises a non-conducting rod20, preferably porcelain or steatite, a thin current-conducting film 21deposited on the surface of the rod 20, end-caps 22 having terminalwires 23 electrically connected to the thin current-conducting film 21,a protective coating 25 and a tubular shield 26 enclosingr thecurrent-conducting surface.

The rod may be made of any suitable material of a ceramic nature such asporcelain or steatite upon which the film 25 may adhere. The lilm 25 isvery thin and is exaggerated in thickness in the drawings. The thincurrent-conducting film 21 is preferably carbon, or metal and may be amixture of carbon and metal, and may include sulphur.

The next general series of steps in my process is to connect theend-caps 22 to the end portions of the thin iilm 21 by depositing anelectrical substance between the current-conducting film and theend-caps Z2 and pressing the end-caps 22 over the electrical substance.The resistor element may or may not be spiraled at 24 depending upon theresistance valve desired.

The next general series of steps consist of providing the coating 25 andthe tubular shield 26 around the resistance element. The protectivecoating 25 is deposited on the resistance film 21 and the protectivetubular shield 26 is mounted around the protective coating 25.

The protective coating 25 comprises a silicone material preferably asilicone resin, and is referred to in the trade as a heat-resistant,water-repellent silicone electrical varnish. More specifically, thevarnish is a phenyl-methyl-silicone resin. In the silicone molecularstructure, the silicon alternates with an oxygen atom so that thesilicon atoms are not bonded to each other. ln a phenyl-methylsiliconeresin, one methyl group and one phenyl group are bonded to each siliconatom. I nd that a silicone resin made by Dow Corning Corporation,Midland, Michigan, and sold under a designation number 994 varnish issatisfactory for my invention. The protective coating 25 is preferablydeposited on the current-conducting lm 21 by spraying thephenyl-methyl-silicone thereon, and then baking the resistor elementwith the phenyl-methyl-silicone deposited thereon for a duration ofapproximately one-half hour at about 400 F. to provide a semi-curedcoating. Another suitable material that may be used to provide aprotective coating 2S is a polysiloxane resin and containsdimethylsiloxane, methylsiloxane, phenylsiloxane, and diphenylsiloxane,and is prepared by the co-hydrolysis and co-condensation of a mixturecontaining dimethyldichloxosilane, methyltrichloxosilane,phenyltrichorosilane, and diphenyl-dichloxosilane. I lind that apolysiloxane resin suitable for my invention may be obtained from UnionCarbide Corporation, New York, N.Y., Silicone Division and sold under adesignation number R-620. The polysiloxane resin R-620 and thephenyl-methyl resin 994 may be used interchangeably.

The tubular shield 26 is in the form of a tube and comprises thefollowing ingredients: initial filler, a reinforcing filler, a siliconerubber gum, benzoyl peroxide, and a Teflon filler powder.

The initial ller that I preferably use in my invention is a nearly pureamorphous diatomaceous silica. Principal minor ingredients includealuminum, iron, calcium and magnesium, usually combined as silicates andnot readily soluble. Water and acid soluble contents are very low. Theaverage particle size is 2-4 microns. A more precise analysis is asfollows:

I nd that an initial ller meeting the above requirements is made byJohns-Manville, Celite Division, New York, N.Y., and is sold under thetrade name of Super- Floss. Hereinafter, this product will be referredto as a silica filler or as a diatomaceous silica.

The silicone rubber gum that I use in my invention preferably comprisesa polysiloxane having dimethylsiloxane, which is prepared by thehydrolysis and condensation of dimethylpoplysiloxane. The siliconerubber gum has a Williams plasticity number of about 70. The siliconerubber gum that is suitable for my invention may be obtanied from UnionCarbide Corporation, New York, N.Y., Silicone Division, and sold under adesignation number W-95.

The reinforcing filler is a hydrated silica of high purity and ofextremely tine particle size. Typical chemical and physical propertiesare:

Color White Bulk density lbs/cu. ft 3 Specific gravity 1.95 Refractiveindex 1.445 Average particle size microns .022 Surface area sq. meters/g 1600 pH in 5% aqueous suspension 4.5 Loss of percent 405 Ignition lossdo 10 S162 ..d0 NaCl do .04 NagSO4 do .04 Oil absorption (linseed oil)gms -160 A suitable reinforcing filler for my invention may be obtainedfrom the Columbia-Southern Chemical Cor poration, a subsidiary ofPittsburgh Plate Glass Company, Pittsburgh, Pa., and sold under thedesignation of "Hi-Sil X303.

A typical analysis of the benzoyl peroxide that I use in my invention isas follows:

A benzoyl peroxide suitable for my invention may be obtained from CadetChemical Corporation, Buffalo, New York, under the designation ofLEenzoyl Peroxide, Puried.

Another chemical additive that is suitable for my invention is Tellon inpowder form. The Teiion that I use in my invention may be obtained fromE. I. du Pont de Nemours & Company, Polychemicals Dept., Wilmington,Delaware.

The process of making the protective tubular shield 26 may be asfollows: I iirst mill in 100 grams of the silicone rubber gum,designation number W-95, with 70 grams of the initial iiller sold underthe designation Super-Floss to obtain a silicone-filler mixture. Then Iallow the silicone-filler mixture of silicone rubber gum and initialiiller to air set for approximately 24 hours.

Next in the series of steps, I mill in 14.6 grams of reinforcing illerwith the silicone-filler mixture to obtain a composite silicone-fillermixture. Next, I mill in 5.3 grams of the benzoyl peroxide powder withthe composite siliconeller mixture containing the reinforcing filler toobtain a suitable mass. Then, the suitable mass is allowed to air setfor approximately 24 hours. Next in the series of steps, I mill in 2 or3% by weight of Teflon in powder form with 189.9 grams of suitable massto obtain a workable mass. Since the amount of Tellon that I use is verysmall, it may be omitted without much alteration in the workable mass.

The workable mass may be extruded into a hollow tube by any suitablemethod using a standard extruding machine in which the extruded tube iscured as it is passed through a heated die at about 155 C. forapproximately five minutes. The hollow extruded tube may be given afurther cure by heating it to about 400 F for approximately liveminutes. The hollow tube after being cured is stretchable as rubber.Ille elongated tube is cut into short sections approximately the lengthof a resistor element. The unstretched internal diameter of the hollowtube is slightly less than the outside diameter of the resistor element.In order to facilitate the insertion of the resistor element into thehollow tube section, one end of the hollow tube section is dipped into asolvent so that the dipped end of the hollow tube section may be easilystretched to a greater diameter than the resistor element. The FIGURE 5shows a convenient way of inserting the resistor element into thestretched end of the tube section. Immediately after the end of the tubehas been dipped into the solvent, the dipped end readily expands,whereby the tube may be easily started over the resistor element byinserting an end-cap 22 into the expanded end of the hollow tu'be 26.After the end-cap 22 has been inserted into the expanded end of thehollow tube 26, the tube is then pulled and stretched over the rest ofthe resistor until it covers the resistor as shown in FIG- URE 1. Whenthe stretched or expanded end of the tube section dries, it will readilyshrink and make a tight fit with the resistor element.

The resistor element with the protective coating 25 thereon is partiallycured so that when the resistor ele- 4l ment with the hollow tube 26thereon is allowed to dry, the tube will shrink and bond itself tightlyto the protective coating 25, and thereby provide a protective shieldfor the resistor element.

The solvent that I use in my invention is preferably toluol. It is alsocalled toluene, methyl benzene, and methyl benzoyl. It is a liquid ofthe composition CGI-LECHE, resembling benzene but with a distinctiveodor. it is obtained as a lay-product from coke ovens and from coal tar.Toluol may be produced by dehydrogenation of petroleum fractions.Another suitable solvent that may be used for dipping the end of aresistor element into is xylene. A similar type of silicone tube whichis chemically and physically about the same as the rubber hollow tubepreviously described in my invention may be obtained from the Bentley,Harris Manufacturing Company, Conshohocken, Pa., and sold under thedesignation Har i258 Extruded Silicone Rubber Tubing.

It is possible to dilate the silicone rubber tubing by immersing it in acommercial solvent such as toluene where in approximately 11/2 minutesit will expand to roughly 11/2 times its original size. The rubbertubing will regain its normal size or conform to the objects it coverswithin approximately 10 to 15 minutes after it has been removed from thesolution of toluene.

A resistance code is provided and is stamped upon the tube section toshow the necessary electrical specications. The resistor is now readyfor final testing and shipping.

Although this invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangement of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

What is claimed is:

l. The process of providing a protective coating and a tubular shield ona resistor element comprising the steps of, providing aphenyl-methyl-silicone resin, spraying said silicone resin on saidresistor element to provide a coating thereon, air setting said resistorelement with said silicone resin coating thereon for one-half hour, thenbaking said resistor element with said coating thereon at approximately400 F. for about one-half hour to provide a semicured coating,constructing a silicone rubber tubing comprising the steps of, providinggrams of dimethyl silicone rubber gum, providing 70 grams of amorphousdiatomaceous silica filler, milling said amount of said silicone gumwith said amount of silica filler to obtain a silicone-filler mixture,air setting said silicone-filler mixture for about 24 hours, providing14.6 grams of hydrated silica and milling said amount of hydrated silicawith said silicone-ller mixture, providing 5.3 grams of benzoyl peroxidepowder and milling said amount of benzoyl peroxide with saidsilicone-liller mixture containing said hydrated silica to obtain asuitable mass, allowing said suitable mass to set for about 24 hours,extruding said suitable mass into a hollow tube having an internaldiameter of the hollow tube slightly less than the outside diameter ofthe resistor element, cutting said hollow tube into sectionsapproximately the length of said resistor element, providing a toluol,dipping one end of a section of said tube into said toluol to stretchthe diameter of said end of said tube section to t'it over said resistorelement, sliding said tube Section over said resistor element and thecoating thereon, allowing the tube section to dry so that said tubesection will shrink and bond itself tightly to said coating and therebyprovide a protective shield for said resistor element.

2. The process of providing a protective shield on a resistor elementcomprising the steps of, providing 100 grams of dimethyl silicone rubbergum, providing 70 grams of amorphous diatomaceous silica filler, millingsaid amount of said silicone gum with said amount of silica iiller toobtain a silicone ller mixture, air setting said silicone-filler mixturefar about 24 hours, providing 14.6 grams of hydrated silica and millingsaid amount of hydrated silica with said silicone-filler mixture,providing 5.3 grams of benzoyl peroxide powder and milling said amountof benzoyl peroxide with said silicone-ller mixture containing saidhydrated silica to obtain a suitable mass, allowing said suitable massto set for about 24 hours, extruding said suitable mass into a hollowtube having an internal diameter slightly less than the outside diameterof the resistor element, cutting said hollow tube into sectionsapproximately the length of said resistor element, providing a toluol,dipping one end of a section of said tube into said toluol to stretchthe diameter of said end of said tube section to t over said resistorelement, sliding said tube section over said resistor element, allowingthe tube section to dry so that said tube section will shrink and bonditself tightly to said coating and thereby provide a protective shieldfor said resistor element.

3. The process of providing a protective coating and a tubular shield onthe surface of a resistor element comprising the steps of, providing aphenyl-methyl-silicone resin, spraying said silicone resin on saidresistor element to provide a coating thereon for one-half hour thenbaking said resistor element with said coating thereon at approximately400 F. for about one-half hour to provide a semicured coating,constructing a silicone rubber tubing comprising the steps of providinga dimethyl silicone rubber gum, providing an amorphous diatomaceoussilica filler, mixing said dimethyl silicone rubber gum with amorphousdiatomaceous silica filler in a ratio of approximately 0.7 to 1.0 toobtain a silicone-ller-rnixture, air setting said silicone-fillermixture for about 24 hours, providing a hydrated silica and mixing samewith said silicone-iiller mixture in a ratio of 14.6 to 170, providing abenzoyl peroxide powder and mixing same with silicone-iiller mixturecontaining said hydrated silica in a ratio of approximately 5.3 to 184.6to obtain a suitable mass, allowing said suitable mass to set for about24 hours, extruding said suitable mass into a hollow tube having aninternal diameter slightly less than the outside diameter of theresistor element, cutting said hollow tube into sections approximatelythe length of said resistor element, providing a toluol, dipping one endof a section of said tube into said toluol to stretch the diameter ofsaid end of said tube section to fit over said resistor element, slidingsaid tube section over said resistor element and the coating thereon,allowing the tube section to dry so that said tube section will shrinkand bond itself tightly to said coating and thereby provide a protectiveshield for said resistor element.

4. The process of providing a protective shield on a resistor elementcomprising the steps of providing a dimethyl silicone rubber gum,providing a amorphous silica ller and milling same with said dimethylsilicone rubber gum to obtain a silicone-filler mixture, providing ahydrated silica and milling same with said silicone-filler mixture,provding a benzoyl peroxide powder and milling same with saidsilicone-ller mixture containing said hydrated silica to obtain asuitable mass, and surrounding said resistoielement with said suitablemass to provide a protective shield for said resistor element, extrudingsaid workable mass into a hollow tube having an internal diameter of thehollow tube slightly less than the outside diameter of the resistorelement, cutting said hollow tube into sections approximately the lengthof said resistor element, providing a toluol, dipping one end of asection of said tube into said toluol to stretch the diameter of saidend of said tube section to iit over said resistor element, sliding saidtube section over said resistor element and the coating thereon,allowing the tube section to dry so that said tube section will shrinkaround said coating and thereby provide a protective shield for saidresistor element.

References Cited in the le of this patent UNITED STATES PATENTS2,460,795 Warrick Feb. 1, 1949 2,634,352 Boykin Apr. 7, 1953 2,660,653Berkelhamer Nov. 24, 1953 2,742,551 Kohring Apr. 17, 1956 2,744,988Tierrnan May 8, 1956 FOREIGN PATENTS 584,549 Great Britain Jan. 17, 1947

1. THE PROCESS OF PROVIDING A PROTECTIVE COATING AND A TUBULAR SHIELD ONA RESISTOR ELEMENT COMPRISING THE STEPS OF, PROVIDING APHENYL-METHYL-SILICONE RESIN, SPRAYING SAID SILICONE RESIN ON SAIDRESISTOR ELEMENT TO PROVIDE A COATING THEREON, AIR SETTING SAID RESISTORELEMENT WITH SAID SILICONE RESIN COATING THEREON FOR ONE-HALF HOUR, THENBAKING SAID RESISTOR ELEMENT WITH SAID COATING THEREON AT APPROXIMATELY400*F. FOR ABOUT ONE-HALF HOUR PROVIDE A SEMICURED COATING, CONSTRUCTINGA SILICON RUBBER TUBING COMPRISING THE STEPS OF, PROVIDING 100 GRAMS OFDIMETHYL SILICONE RUBBER GUM, PROVIDING 70 GRASMS OF AMORPHOUSDIATOMACCEOUS SILICA FILLER, MILLING SAID AMOUNT OF SAID SILICONE GUMWITH SAID AMOUNT OF SILICA FILLER TO OBTAIN A SILICONE-FILLER MIXTURE,AIR SETTING SAID SILICONE-FILLER MIXTURE FOR ABOUT 24 HOURS, PROVIDING14.6 GRAMS OF HYDRATED SILICA AND MILLING SAID AMOUNT OF HYDRATED SILICAWITH SAID SILICON-FILLER MIXTURE, PROVIDING 5.3 GRAMS OF BENZOYLTEDPEROXIDE POWDER AND MILLING SAID AMOUNT OF BENZOYL PEROXIDE WITH SAIDSILICONE-FILLER MIXTURE CONTAINING SAID HYDRATED SILICA TO OBTAIN ASUITABLE MASS, ALLOWING SAID SUITABLE MASS TO SET FOR ABOUT 24 HOURS,EXTRUDING SAID SUITABLE MASS INTO A HALLOW TUBE HAVING AN INTERNALDIAMETER OF THE HALLOW TUBE SLIGHTLY LESS THAN THE OUTSIDE DIAMETER OFTHE RESISTOR ELEMENT, CUTTING SAID HALLOW TUBE INTO SECTIONSAPPROXIMATELY THE LENGTH OF SAID RESISTOR ELEMENT, PROVIDING A TOLUOL,DIPPING ONE END OF A SECTION OF SAID TUBE INTO SAID TOLUOL TO STRETCHTHE DIAMETER OF SAID END OF SAID TUBE SECTION TO FIT OVER SAID RESISTORELEMENT, SLIDING SAID TUBE SECTION OVER SAID RESISTOR ELEMENT AND THECOATING THEREON,, ALLOWING THE TUBE SECTION TO DRY SO THAT SAID TUBESECTION WILL SHRINK AND BOND ITSELF TIGHTLY TO SAID COATING AND THEREBYPROVIDE PROTECTIVE SHIELD FOR SAID RESISTOR ELEMENT.